The fluid catalytic cracking (FCC) process has become well-established in the petroleum refining industry for converting higher boiling petroleum fractions into lower boiling products, especially gasoline.
In the fluid catalytic process, a finely divided solid cracking catalyst promotes cracking reactions. The catalyst is in a finely divided form, typically with a particles of 20-100 microns, with an average of about 60-75 microns. The catalyst acts like a fluid (hence the designation FCC) and circulates in a closed cycle between a cracking zone and a separate regeneration zone.
In the cracking zone, hot catalyst contacts the feed so as to effect the desired cracking reactions and coke up the catalyst. The catalyst is then separated from cracked products which are removed from the cracking reactor for further processing. The coked catalyst is stripped and then regenerated.
A further description of the catalytic cracking process may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts", Venuto and Habib, Marcel Dekker, N.Y., 1978, incorporated by reference.
Although the FCC process has been around more than 50 years, there are still many problem areas. A significant problem is poor stripping. The conventional stripping, in a single stage, by counter-current contact with steam, leaves a lot of cracked product adsorbed on or entrained with spent catalyst. From 10 to perhaps 40 to 50% of the material burned as coke in the regenerator is potentially recoverable hydrocarbon. Thus much work has been done to improve stripping, ranging from long residence time strippers to hot stripping designs.
U.S. Pat. No. 4,481,103, Krambeck et al taught conventional stripping followed by another 1-30 minutes of stripping at moderate temperature.
U.S. Pat. No. 4,789,458 Haddad et al taught an FCC process with a conventional stripper followed by a hot stripper. Hot stripping was achieved by adding some hot regenerated catalyst to the catalyst discharged from the conventional stripper.
Various other stripper arrangements have been proposed, including cyclonic strippers directly connected to an FCC riser reactor outlet.
We looked at these stripping approaches, and were concerned that none provided the optimum solution. Most of these approaches to stripping started with conventional steam stripping, wherein 1 to 5 wt % steam contacts spent catalyst discharged from a riser reactor. They usually then tried to improve stripping by heating the catalyst, or stripping it longer. We believed that it was important to quench and cool the catalyst, rather than heat it, as the first step. This would be considered a step backward by most FCC experts, in that the conventional wisdom is that hot stripping is better stripping. While this is true from a strict diffusion limited view of stripping, it ignores the complex and interconnected activities that go on in a conventional stripper.
Coke on FCC catalyst is associated to the CCR content of the feed stock and to the catalytic chemistries occurring in the FCC. The quenched stripper concept addresses minimizing catalytic coke formation in the stripper; whereas, the hot second zone of the stripper addresses removal of both CCR coke and catalytic coke.
Though no definitive reaction pathway for coke formation has been developed, numerous well accepted factors affect the rate of catalytic coke formation: reaction temperature and time, the nature of the catalyst, the partial pressure of the oil/coke precursors, and the type of oil/coke precursors. Higher reaction temperature will enhance the formation of coke if the coke precursors are present, and will continue to dehydrogenate soft coke to hard coke. Numerous catalyst functionalities affect the rate of coke formation, e.g. dehydrogenation activity via contaminate metals such as Ni, V, Fe; hydrogen transfer ability of the catalyst; and the concentration and strength of Lewis and Brondsted acid sites. The hydrocarbon type also affects coke formation. Hightower and Emmett [J. W. Hightower & P. H. Emmett, J.Am.Chem.Soc., 87; 939 (1965)] found olefins to have a much higher propensity for coke formation than paraffins or aromatics of nearly equivalent molecular weight. Light olefins and aromatics are the predominate FCC product hydrocarbons found in the stripper, thus minimizing their reaction rate to coke formation is of primary importance.
We seek to improve stripper performance by first removing the oil/coke precursors from the interstitial void volume of the stripper at a lower temperature than conventional FCC stripping. The lower temperature reduces the catalytic condensation reactions responsible for catalytic coke formation. This reduces the formation of coke in the stripper relative to conventional higher temperature stripping. We avoid reducing the temperature below 900.degree. F. at the top of the stripper, as this could condense heavy oil products on the catalyst.
We discovered that cool stripping of catalyst, followed by a hot stripping stage, produced significantly less coke than the conventional steam stripping, and much less than hot stripping.